Q. Can a person really know whether he/she will go to Heaven when they die?
A. Yes, because God communicated His Word (the Bible) to us so we may know His requirements for going to Heaven.
 "These thing have I written unto you that believe[ing] on the Name of the Son of God (Jesus Christ); that ye may know that ye have (present tense possession) eternal life..." (1 John 5:13)

Q. What is God's Law?
A. God's Law is that perfect standard of righteousness (right acting and right thinking) that is summed up in the Ten Commandments and the Two Commandments of Jesus Christ.
The Ten Commandments
Exodus 20:1-17 "And God spake all these words, saying,
#1. "Thou shalt have no other gods before Me."
#2. "Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth:   Thou shalt not bow down thyself to them, nor serve them: for I the Lord thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate (reject) Me; And shewing mercy unto thousands of them that love (receive) Me, and keep my commandments."
#3. "Thou shalt not take the Name (representing all that God is) of the Lord thy God in vain; for the Lord will not hold him guiltless that taketh His Name in vain."
#4. "Remember the sabbath day, to keep it holy. Six days shalt thou labour, and do all thy work: But the seventh day is the sabbath of the Lord thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: For in six days the Lord made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the Lord blessed the sabbath day, and hallowed it."
#5. "Honour thy father and thy mother: that thy days may be long upon the land which the Lord thy God giveth thee."
#6. "Thou shalt not kill (murder)."
#7. "Thou shalt not commit adultery."
#8. "Thou shalt not steal."
#9. "Thou shalt not bear false witness against thy neighbour."
#10. "Thou shalt not covet thy neighbour's house, thou shalt not covet thy neighbour's wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is thy neighbour's."
(The Ten Commandments summed up in Jesus Christ's Two Commandments) Mark 12:30-31, Jesus speaking,
#1. "And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment."
#2.  "And the second is like, namely this, Thou shalt love thy neighbour as thyself. There is none other commandment greater than these."

Q. Where did sin come from?
A. When God created Adam and Eve as the first human beings, and placed them in the Garden of Eden, God denied them access to one tree while giving them freedom over every other thing in the Garden (Gen. 2:16-17).  The tree denied was the tree of the knowledge of good and evil.  Adam and Eve were created innocent, but not incapable of sin.  They were also created with the ability to choose to obey God or to disobey God.   God warned them that "in the day that you eat of that tree you shalt surely [spiritually] die." 
But Satan said,
"Has God said?  For God knows that when you eat of that tree, your eyes shall be opened, and you shall be as gods, knowing good and evil."
As long as Adam and Eve remained in innocence they would never die physically or spiritually.  But by disobeying God they lost their innocence, died spiritually, and became guilty sinners before God, responsible for the breaking of God's law.
"Eve saw the fruit of the tree; Eve desired to be wise, she took of the fruit, gave also to her husband, and Adam did eat" (Genesis 3:6).   "And Adam was not deceived, but the woman being deceived was in the transgression" (1 Timothy 2:14).

Q. How does sin affect me?
A. Since Adam and Eve are not only the first humans, but also the first parents, every human being traces their lineage directly back to them.   Every person is genetically bound to Adam and Eve.  Therefore their "fall" (disobedience to God), which resulted in their "fallen nature" (sin nature) has been passed down through the centuries through each set of parents to their children, and ultimately to you.  You were born with a sin nature that causes you to sin, therefore you are a sinner in actuality and in the sight of God. 
"Wherefore, as by one man (Adam) sin entered into the world, and death ("separation from God") by sin; and so death passed upon all men, for that all have sinned" (Romans 5:12)

Q. So I am a sinner like every other man or woman.  What does that matter?
A. Because God is a holy, righteous God who cannot and will not allow sin in His presence, therefore your sin-nature and the resulting sins will keep you from ever being in God's presence.  In other words, as a sinner you cannot be permitted to enter Heaven when you die.  And since your soul never dies, you will spend eternity in Hell.
"Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power"  (2Thessalonians 1:9)

Q. Though I may be a sinner before God in the strict sense, yet I am not as wicked as a murderer or a thief. Why am I to be punished in Hell like a murderer or a thief?
A. Because in God's eyes, to break one of His Laws, no matter how "minor" in your eyes, is to break each of His Laws.  When you broke the "least" of His Laws you revealed your disobedience toward God, and you will be held accountable for that disobedience. 
"For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all." (James 2:10)
"For as many as are of the works of the law are under the curse: for it is written, Cursed is every one that continueth not in all things which are written in the book of the law to do them." (Galatians 3:10)

Q. Ok, as a sinner, what is my position before God?
A. In your thinking processes you are an enemy toward God. You give not a thought toward God, and if you do think about God it is a god of your own making and choosing.
"Because the carnal ("earthly nature") mind  is enmity ("hostile") against God: for it is not subject to the law of God, neither indeed can be."   (Romans 8:7)
A. Because of your sin-nature and your resulting sins, you are already condemned before God even as you live.
"He that believeth on Him [Jesus Christ] is not condemned: but he that believeth not is condemned already, because he hath not believed in the Name of the only begotten Son of God." (John 3:18)
A. Your mind is darkened by the god of this world, Satan.
"In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, Who is the image of God, should shine unto them." (2 Corinthians 4:4)
A. You live, move and have your being in darkness.
"And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil." ( John 3:19)
A. Though you have physical life, you are dead spiritually.
"And you, being dead in your sins and the uncircumcision of your flesh..."   (Colossians 2:13)
 "And you ... who were dead in trespasses and sins" (Ephesians 2:1)
This is best pictured by a great chasm or divide.  You are on one side and God is on the other.  There is no human or earthly way for you to gain the other side.  Nothing you can or would do is sufficient to bridge the gap.

Q. How can I get from where I am to where I need to be?
A. First, you need to realize and acknowledge that you are a helpless and hopeless sinner, and that the great divide between you and God is of your own making. 
Second, you need to realize that you have offended a holy and righteous God by your actions and disobedience. 
Third, you need to realize that God is the One who seeks to reconcile you to Himself.  God does not need you to be reconciled to Himself, but His great mercy, grace and longsuffering are purposefully made available to sinners.
Fourth, you need to realize that the God who seeks to reconcile you has revealed His plan and purpose to you through His written Word, the holy Bible.  The Bible is the only source of truth concerning God, and your hope for going to Heaven when you die.  And only the truth can set you free.
Fifth, You need to believe ("put your trust in the word of Another") that what God says is true. 
Sixth, You need to believe that Jesus Christ is God come in human flesh to reveal God to mankind.  You need to believe that Jesus Christ died on the cross of Calvary to satisfy the wrath of God toward you and your sins.
Seventh, You need to repent ("change your mind") about who God is, and how offensive your sins actually are.  You need to see yourself as God sees you, a lost, undone sinner in need of a Saviour.
Eighth,  You need to believe that the death Christ died was sufficient to pay the price of your sins, and that His resurrection from the grave is proof that God accepted Christ's shed blood on your behalf and in your place.
Ninth. You need to receive the free gift of salvation as offered by God through Jesus Christ.

Believe—Repent—Pray,
“God be merciful to me a sinner who believes in my heart that Jesus Christ paid the full payment of my sin; that He fully satisfied the wrath of God on my behalf through His shed blood; and that God raised Him from the dead, fully satisfied with His sacrifice. I now repent of my sins and trust Jesus Christ’s death alone for my salvation. In Jesus name I pray, amen.” (John 3:16) 

“For whosoever shall call upon the name of the Lord, shall be saved” Romans 10:13